Herbert grove dorset



OctS, 1929. H. G. DORSEY FILTER Filed Nov. 11,3192? Patented Oct. 8,1929 UNITED STATES4 PATENT OFFICE HERBERT GROVE DORSEY, OF GLOUCESTER,MASSACHUSETTS, ASSIGNOB TO SUB- MARINE SIGNAL COMPANY, OF PORTLAND,MAINE, A .CORPORATION OF MAINE rILTER Application tiled November 11,1922. Serial No. 600,350.

The object of this invention is to eliminate noises in submarinesignaling. In the art of submarine signaling it is usual to employmicrophones or magneto telephones, now commonly known as hydrophones,which are mounted upon orA towed from a ship or similar vessel at apoint which will be at all times Well below the surface of the Water inwhich such vessel is floating. It

has always been found in the past that hydrophones mounted upon a movingvessel were subject to excessive noises created in the hydrophones notonly by the machinery upon' rangement or association with each other,

these disturbing noises are liable to be so loud as to completelyoverpower the sounds ofthe submarine signals produced by a distantsource. In order to reduce these disturbing ship and water noises it hasbeen necessary in the past to mount the hydrophones in tanks Within thevessel in the manner described in U. S. Patent No. 768,570 although thisreduces somewhat the sensitiveness of the hydrophone. Another method hasbeen to use a large number of hydrophones so arranged and connected thatthese disturbing noises are suiiiciently out of phase with each other atthe several hydrophones to produce an interference which reduces theirintensity and thus permits the submarine signals to be more easilyheard.

Submarine signals have in the past been most commonly created by bellsstruck under water or by electromagnetic devices such as theFessendenoscillator (U. S. Patent No. 1,167,366) Both of these types ofsubmarine sound producers create submarine sound waves of a Well deneddominating frequency, their difference-being that the sound Wavescreated by the submarine bell are highly damped While those from theoscillator are undamped when heard at any deinite distance from theoscillator. Innumerable attempts have been made to take advantage ofthis uniformity in the frequency of the sound Waves created by submarinesignaling devices by using mechanical means which it Was hoped would beresonant at the frequency of the source of the signals and consequentlynon-resonant at other frequencies. Water and ship noises are composed oftones of various frequencies and it was hoped that by some such meanssuch noises would be reduced in intensity at. the receiver While thesubmarine sig'nals would be increased in intensity. None of thesedevices has been commercially successful, partly owing to the -fact thatit is difficult to maintain the desired pitch in a resonant system ofwhich one element is a microphone, and more particularly for the reasonthat there were often tones in such noises of the pitch of the-submarinesignal source to cause the water and ships noises to energize theresonant system and cause tones of considerable energy of the same pitchas that of the submarine signal source. This resonant system thereforefailed as false signals were liable to be created. Another method whichhas been tried has been the use ofelectrical filters between thehydrophones andthe telephone receivers. These electric filters consistedof condensers and inductance coils so arranged and of such proportionsthat the circuit was resonant only to currents of the frequency of the'source of submarine signals. Filters of this kind have never been foundpracticable partly for the reason that the apparatus added to thecircuit tended to reduce the intensity of the signal but moreparticularlyfor the same reason that mechanically resonant systemsfailed, that is to say, for the reason that the noises, produced by theship or by the 'Water inthe vicinity of the hydrophones create -byimpulse action on the hydrophone some tones of the same pitch as that ofthe submarine signals and, as a consequence, make it impossible todistinguish the submarine signal.

I have invented a filter which in service under most adverse conditionsentirely removes the objections inherent in resonant systems whethermechanical or electrical or in the filters hitherto employed. Thisfilter comprises two thermionic tubes each con! nected with 'one of twointerconnected re-Y generative circuits. It has been possible by thesemeans to suppress water and other disturbing-noises and permit the tonesof submarine signals to be heardwith relatively great power orintensity. It has been found possible to give such proportions to theelectrical devices as far as inductance, resistance and capacity areconcerned that all disturbing sounds other than low audible frequenciesare filtered out.

In efforts to malie a filter of this character I have tried a veryconsiderable number of circuits and differently proportioned circuitswithout getting satisfactory results, and I. have described below acircuit which I have found very effective, though my invention may beotherwise embodied as Will be apparent to those skilled in the art.

The effectiveness of the present type of circuit for detecting signalsin a medium agitated by disturbing noises and other compressionalimpacts due to the motion of the water or otherwise is due it isbelieved to the peculiar characteristics of the circuit. Tt is wellknown that circuits which are highly tuned and have only slight damping'are very liable to be affected by so-called impact excitation whichsimply acts in such a manner as a bell would act when struck with ahammer; in other Words, the impact whether it has a characteristicfrequency or not sets the circuit in oscillation at the naturalfrequency of the circuit.

In the present system this is avoided by providing a circuit which ishighly timed and slightly damped., notat all times but only periodicallyat such intervals as controlled by some external means.

In the present invention a three-electrode vacuum tube is used, havingplate and grid circuits wherein some of the energy of the plate circuitis fed back to the grid circuit for the purpose of regenerativeamplification. By feeding back sufficient energy from the plate to thegrid circuit it is well known that such a system can be made to continueits oscillations indefinitely without the aid of any signal. At thepoint'where such a system just begins to oscillate the resistance andtherefore the damping of the system may be considered as negligible.When the system is made to oscillate, the resistance or damping isthought of as being of a negative valine and therefore energy issupplied to the system. Under the latter conditions a tuned circuit suchas described in the present specification will, of course, have nodamping at all and will tend to maintain itself in oscillationindefinitely. When an impact therefore is applied to a. circu`it underthese conditions this impact may easily `start the system oscillating in'such a manner as to be maintained indefinitely. However, in thelpresent invention such continuous oscillations by impact excitation isavoided by increasing periodically the amount of energy absorbed by thegrid circuit so that the circuit becomes highly damped. Therefore anyimpact which might have a tendency to maintain the circuit inoscillationwould periodically be damped out by increasing the energyconsumed in the grid circuit since in effect the system then becomes ahighly damped system.

In the accompanying drawings is shown a diagram of connections for animproved circuit and in which a receiving microphone 10 is connected inseries with a battery 11 and a winding 12 which forms a primary coil ofa transformer T1. This winding 12 might be lconnected to a magnetophoneinstead of the microphone 10 and in general is connected to anyinstrument which may receive the compressional waves or other soundsignals which are to be used in the work and generate an electricalcurrent or cause variations of current corresponding to the compressional wave impulses received. Transform er T1 is made with a laminatediron core and has also a secondary coil 13, the outside end of which isconnected by wire 130 to the plate 15 of the thermionic valve B and theinside end by wire 131 to the positive side of battery 20. The tertiarycoil 14 of the transforirter Tl has the inside end of its windingconnected by wire 140 to thc grid 16 of the thermionic valve B and theoutside end of its winding by wire 141 to the filament 17, this side ofthe filament being also connectedto the battery .18, whose positive sideis connected to the negative side of the plate battery 20. The secondarywinding 13 has one end connected to the'plate 15 of the thermionic tubeand the other end to the positive side of the battery 20. The primarywinding 12 is connected across the microphone 10 in series with thebattery 11. The other side of the filament 17 is connected totherheostat 19, which is connected to the master rheostat 21, which isconnected through the ammeter 22 to the negative side of 'the battery18.

Across the terminals of the tertiary winding 14 is arranged a switch arm35 so that it may be connected to any one of the condensers 32, 33,` 34,and there is also connected across those terminals a variable condenser31.

The grid 16 of the thermionic valve B is also connected by wire 23 tothe secondary wind ing 36 of the laminated iron core transformer T2, andthis winding is shunted with a variable condenser 37. The inside end ofthe winding 36 is connected by wire 24 to the grid 38 of thermionievalve C which has one side of its filament 39 connected by wire 25 tothe positive side of the battery 18, the other side of the filamentbeing connected through the variable resistance 51 and the masterrheostat 21 and ammeter 22 to the negative side ofbattery 18. The plate40 of the thermionic valve C is connected to the `grid through thevariable condenser 53 and the variable inductance 52 and the plate isalso connected bv wire 26 to the outside end of the winding 54 of thetransformer T2. The inside end of said winding 54 being connected bywire 27 to one side of the telephone 55, the other side of which isconnected by wire 28 to the battery 20. A condenser 56 is bridged acrossthe telephone 55 and plate battery 20. It should be understood that Irefer to inside and outside ends of the windings of transformer T1 andT2 to show that there is to be a regenerative action in these 'twotransformers and that they must be properly connected orregeneration'will not take place.

To those versed in the art it will be readilyV noticed that theconnections might be reversed by pairs and the regenerative action stillmaintained.

The operation of this circuit will now be described. In terms oftelephone coil coupling there is very close coupling between the coils13 and 14 and very loose coupling between these two and the coil 12. Intransformers built for actual work the coil 14 is wound with about 1200turns of #24 enameled. wire. having an inductance of 95 millihenries andcoil 13 is Wound with about 1250 turns of #24 enameled Wire having aninductance of about 133 millihenries, the two coils having a coefficientcoupling of .99. The very loose coupling between the coils 12 and 13 maybe obtained in any suitable manner known in the art.

Considering the circuit composed of the thermionic tube B, the twobatteries 18 and 20, the transformer T1 and the condensers across itsterminals, the regenerative action between the coils 13 and 14 willcause violent singing of the system at a frequency depending upon whichof thecondensers 32, 33 and 34 are connected across the terminals ofcoil 14 by the switch 35. These condensers are so adjusted that theywill tune the system to any three standard fixed frequencies, such as540 cycles, 1050 cycles and 1215 cycles, being for the coil values abovementioned about .73 mic'ofarads, .20l m.f. and .10 m.f. respectively,these being common frequencies used with different submarine signalingoscillators and bells. The variable condenser 31 is used to produceslight changes in tuning to meet the conditions of any individualsubmarine signal sender.

Considering now the second part of the circuit, namely, the thermionictube C, transformer T2, the batteries 18 and 20 and the telephone 55,the regenerative action in the transformer T2 will produce an electricaloscillation in the circuit depending upon the constaluits` ofthetransformer and the condenser 37, as. welljasthe variable inductance 52and the variable condenser 53. Good results have been obtained by havingthe transformer T2 made of 2400 turns of #34 enameled wire for theprimary coil 54 and of 8000 turns of :#:28 enameled wire for the coil 36and having a capacity of 1 m.f. for the condenser 37. The frequency ofthe oscillations of the first tube feeding back on thc grid of y thesame tube in the well known regenerative method and is adjusted andtuned so that the greatest variations of the grid potential will besymmetrical with a point on the above mentioned characteristiccurve,-Which is the middle of the straight portion of the curve. Puremaximum amplification of the signal may then take place without an;7distortion and without rectification. The oscillations of the incomingsignal are passed on to the second tube similar in wave formv to whatthey were when received but ampli fied. If the self oscillations set upin the first tube, however, by reason of the operation ofthe tube at theApoint in the characteristic curve mentioned above, are allowed tocontinue without interruption the ability of the tube to amplify isgreatly diminished.` If the tube could remain in a state so that eachimpressed oscillation due to the incoming signal would force the tube tobuild up oscillations, a maximum amount of amplification would result.The oscillations due to the signal would in this way be regenerated overand over to the maximum capacity of the tube from practically aninoperative state of the tube, in contradistinction to the regenerationfrom the self oscillating state to a maximum oscillating state as inthecase of simple regeneration.

By increasing the filament current in the second tube which is tuned toa frequency of about 5 to 12 vibrations per second, these low frequencyvibrations are impressed through the feed back system, transformers T2and T1 upon the grid of the first amplifier varyingl periodically thegrid potential of the first amplifier. By varying the grid potential ofthe first tube in this way, the self oscillations of the tube are notgiven a chance to build up and sustain themselves because during certaintimes, though relatively short as co-mpared to the total periodicvariation, the point of openation of the tube is at the ends of thestraight portion of the characteristic curve its fullest capacity.

Where `the self oscillations of the tube cannot Well exist.

The action of the circuit is then as follows The incoming signals arereceived by the detector, microphone or any suitable means and act tovary the electrical oscillations in l the coil 12. These electricaloscillations, transformed by the transformer T, act on the Grid 16 tovary its potential, thereby chang ing the plate 15 current-accordingto'the variation of the grid potential 16.- The action of the coil 13 ofT then feeds these oscillations back upon the grid 16 which processcontinues until the tube has amplified to At the same time the tunedfilament grid circuit is partly tuned to the incoming signals, therebyeliminating some of the disturbing noises. The oscillations 'of thefirst tube are passed on to the second tube through the transformer T2and in a similar Way acting upon the grid 38 are thereby amplified. Thelocal circuit 40, 53, 52, 24, 38 is tuned to the incoming signal thuseliminating all the disturbing noises. The signals Without other noisesmay then be heard plainly in the telephone 55. During this process thesecond tube has the other function of feeding back sub-audibleoscillations on the first tube to keep the firsttube at the point Wherethe incoming signals will cause the tube to commence to oscillate. Thislast process is sometimes. called superregeneration.

Now, if the circuit for tube B is operated alone, it will sing at say afrequency of 1050 cycles. Ifthe second tube C is then put in circuit andthe filament current gradually made, stronger and stronger a point willbe reached when the tube B will cease to oscillate,but there Will stillbe oscillations in the tube C. At this point incoming signals will beamplied and noise suppressed, but will be varied or modulated at thefrequency of the oscillations of the. tube C and, 'in some form of work,this is not objectionable and in o\ further adjustments need be made.However, for certain other forms of submarine ,signaling reception,especially with bell signals, it is desirable to have the signals comein as nearly as possible with their own characteristics, and to reachthis condition the filament current in tubes B and C. are increased insteps, first causing B to oscillate and then destroying'the oscillationsby in'- creasing the filament current in C, and at the same timecondenser 53. and inductance 52 are adjusted until a condition isreachedin which incoming signals are received with almost their owncharacteristics and are greatly amplified, While any noises are entirelysuppressed unless theybe of such a nature that they continue to excitethe system by impulses sustained at exactly the frequency at .which itis tuned.. However, thislatter condition is almost never reached by thenoises one encounters in submarine signaling Where one is dealing withfrequencies in the neighborhood of from about 200 to 2000 cycles persecond.

Vhat I claim as my invention is 1. A. system for receiving substantiallyundamped compressional-wave disturbances comprising detector means forconverting compressional .wave energy into electrical energy, a normallydeenergized tuned circuit adapted to continue tol oscillatc when setint-o oscillation by shock, means for damping said circuit periodicallyat a non-audible frequency andl means coupled to said tuned circuit forreceiving signals of undamped audible frequency.

2. A system for, receiving substantially uudamped eompressional Wavedisturbances comprising sound translating means for convertingcompressional wave energy into eleccircuit periodically at anonsau'dible fre,

quency vincluding a local oscillatory circuit and -means for impressingpotential varia-v tion therefrom on the gridv of said vacuum tube atsaid non-audible frequency and means coupled to said tunedcircuitforreceivmg signals of undamped audible frequen C Q HERBERT GROVEDORSEYY.

